End-loaded Split Test Research Articles

A double compliances method for determination of the ELS mode II interlaminar fracture toughness without measuring the growing crack length

Due to crack closure, determination of the interlaminar mode II fracture toughness of composite materials is more challenging than that of mode I. Three data reduction methods (SBT, ECM and CBTE) are recommended in the ISO standard 15114 to calculate the interlaminar mode II fracture toughness using End-Loaded-Split (ELS) specimen. However, the maximum difference in the fracture toughness obtained from these methods is up to 20%. A double compliances method is proposed to determine the interlaminar mode II fracture toughness of an ELS composite specimen, without measurement of the crack growth length. Both virtual and real ELS tests are conducted to verify the accuracy of the present method. It is found that the fracture toughness obtained from the present double compliances method and CBTE are consistent, whereas the crack growth length calculated from the present method is more accurate than that from CBTE. The SBT is very simple, but its accuracy is fair for calculation of the fracture toughness.

An Open-Source ABAQUS Plug-In for Delamination Analysis of 3D Printed Composites.

This article presents the development and implementation of the Delamination Plug-in, an open-source tool for modeling delamination tests in the ABAQUS software. Specifically designed for stochastic modeling of 3D printed composites, the plug-in combines the benefits of the graphical user interface (GUI) and the programming of commercial finite element (FE) software. The Delamination Plug-in offers an effortless alternative to the time-consuming analytical modeling and GUI work involved in delamination tests and includes algorithms for several tests, such as the double cantilever beam, end-loaded split, end-notched flexure, and modified end-loaded split tests, solved using the virtual crack closure technique and the cohesive zone method. It enables the user to develop simulations for both simple symmetric laminates and generally layered laminates with additional thermal stresses. The applicability of the tool is demonstrated through its use in two distinct delamination problems, one for conventional and one for 3D printed composite laminates, and its results are compared to analytical models and experimental data from the open literature. The results demonstrate that the Delamination Plug-in is efficient and applicable for such materials. This establishes the tool as an important means of automating delamination analysis and for the development and testing of 3D printed composites, making it a valuable tool for both researchers and industry professionals.

A semi-analytical method for the determination of fracture toughness and bridging law in ELS test

Fracture toughness and bridging law are necessary for characterizing the delamination behavior with the effect of fiber bridging. Accurate determination of them in a simple way is particularly important for the design of composite structures. The end-loaded split (ELS) method, which can obtain stable delamination growth, has been applied in the ISO 15,114 standard to measure the mode II fracture toughness. However, geometric nonlinearity and difficulty on the crack observation are unavoidable. To deal with those problems, a semi-analytical method is proposed in this study for determining the fracture toughness and the bridging law. Predictions from the semi-analytical method are consistent with experimental results of CFRP and GFRP laminates. Furthermore, numerical modelling is performed based on a tri-linear cohesive zone model which integrates the determined bridging law. Numerical load–displacement responses have good agreements with the experimental ones, which illustrates the applicability and accuracy of the proposed method. The proposed method overcomes the problem of real-time monitoring of the crack growth length and the relative sliding displacement at the pre-crack tip, and considers the correction factors to solve the nonlinear problem caused by the large displacement, which is simple and efficient for practical application.

Configuration of the ELS test for the determination of the mode II fracture toughness in adhesive joints

In order to obtain Mode II fracture toughness of unidirectional fiber composite materials, the End-Loaded Split (ELS) has enjoyed great success because of its higher stability. As a result, crack growth initiation and propagation values are obtained, the latter being especially used in interlaminar progressive damage models. The propagation values must be obtained when the Failure Process Zone (FPZ) has completely developed. In delamination tests of unidirectional composites, such as those contemplated in ISO 15114, the dimension of the FPZ can be neglected against the rest of the dimensions of the specimen, whereby the self-similar crack growth is reached just after the initiation of the crack propagation. However, in adhesive joints, the dimension of the FPZ increases significantly due to the plasticity of the adhesives. And even it can be larger than the entire length reserved for crack propagation. When this occurs, only initiation values are obtained, always inferior to the values of propagation, being these values too conservative to reproduce the behaviour of the joint. In this paper we present a methodology to define the configuration of the ELS test and the specimen dimensions required to obtain propagation results in adhesive joints. The methodology defined is based on two criteria that take into account the length of the FPZ and the stability of the test. As a result, a domain of validity of the test is obtained, which, for a certain type of adhesive and specimen configuration, it indicated is there will be propagation or not.

Embedding artificial neural networks into twin cohesive zone models for composites fatigue delamination prediction under various stress ratios and mode mixities

This paper presents for the first time a novel numerical technique for modelling fatigue delamination growth in fibre reinforced composites, which is based on coupling two twin cohesive zone models with a single-hidden-layer artificial neural network. The simulation approach proposed here can describe composites fatigue delamination under negative & positive stress ratios and the full range of mode mixities. In the modelling strategy, each segment of a composites interface is described by two twin cohesive elements, which jointly provide local fracture mechanics parameters into a feedforward single-hidden-layer neural network, without the need to know the global load R ratio. In turn, the neural network algorithm feeds the fatigue crack propagation rate da/dN back into the twin cohesive elements, which follow a static and fatigue cohesive law in a synchronous fashion. The novel modelling methodology has been implemented in an explicit finite element scheme. The modelling strategy is first verified and validated by several benchmark cases, involving mode I Double Cantilever Beam tests, mode II End Loaded Split tests with and without reversal, as well as Mixed-Mode Bending tests. A relevant application of the modelling technique is demonstrated considering a tapered laminate, which experiences non-proportional loading due to the presence of combined static tension and cyclic bending.

Effect of environment conditioning on mode II fracture behaviour of adhesively bonded joints

The effect of environmental conditions on the fracture behaviour of two types of adhesively bonded joints (wet-aged and non-aged) is experimentally studied under mode II loading. End Loaded Split tests were performed at various temperatures (−55 °C, room temperature (RT) and 80 °C) on non-aged and aged specimens. The non-aged specimens were stored in a laboratory under controlled conditions at RT (23 °C/55 RH) while the wet-aged specimens were exposed to accelerated ageing in an environmental chamber at 70 °C/85% RH for four years. The data reduction was applied using an inverse method and two other direct methods. A good agreement between these methods is observed. The analysis of the obtained results shows that extremely long aging times considerably affect the fracture response of bonded joints under mode II (the fracture toughness of the wet-aged specimens degraded by more than 30%). Furthermore, testing at high or cryogenic temperatures has a significant effect on the fracture response. Indeed, testing at high temperatures was observed to increase the fracture toughness (this increase ranged from 8% to 80%) while cryogenic temperatures increased the brittleness of the adhesive which caused a substantial reduction (more than 25%) in the fracture toughness of the bonded joint.

Theory of dynamic mode-II delamination in end-loaded split tests

The dynamic mode-II energy release rate (ERR) of the end-loaded split (ELS) test with applied time-dependent displacement is derived for the first time with the effect of vibration included. Dynamic Euler-Bernoulli beams are used together with a deflection condition to simulate contact. To understand the dynamic effect and the relative dynamic contribution from each vibration mode, a ‘dynamic factor’ and a ‘spatial factor’ are defined. It is found that the contribution of the ith vibration mode is dependent on the spatial factor, which is a function of the delamination length and the total length of the ELS specimen. Certain vibration modes are dominant dependent on the spatial factor. In addition, for a given spatial factor, there may be a certain vibration mode, which makes approximately zero contribution to the ERR. The developed theory is verified against results from finite-element method (FEM) simulations and it is in excellent agreement. This work now allows the loading rate-dependent mode-II delamination toughness of materials to be determined by using ELS tests. In addition, it provides understanding of the structural dynamic response in the presence of mode-II delamination and can guide the design of structures to mitigate against vibration-driven delamination.

An Investigation of the End-Notched Flexure and End-Loaded Split Tests Applied to the Mode II Interlaminar Fracture of a SiC/SiC Ceramic Matrix Composite

Abstract Delamination is a common failure mode observed in ceramic matrix composites (CMCs) and occurs as a result of applied interlaminar tensile and shear stresses exceeding the interlaminar strength. As CMCs are further implemented into aero engines, the need to understand their interlaminar failure becomes increasingly important. While significant contributions have been made toward understanding the mode I fracture toughness of CMCs, limited work exists on mode II. Several test methods for measuring the mode II fracture toughness have been proposed in the literature, namely, the end-notched flexure (ENF) and the end-loaded split (ELS) tests. This work investigates the mode II fracture toughness of a melt-infiltrated SiC/SiC CMC at ambient temperature using the ENF and ELS test methods. Acoustic emission (AE), direct current potential drop (DCPD), and digital image correlation (DIC) are implemented as health monitoring techniques to monitor crack initiation and propagation. Results show reasonable correlation between the two test methods and that the ELS test method is better suited for characterizing R-curve behavior.

Mode-II fracture behaviour of aerospace-grade carbon fibre/epoxy composites interleaved with thermoplastic veils

Thermoplastic veils based on Polyethylene-terephthalate (PET), Polyphenylene-sulfide (PPS) and Polyamide-12 (PA) fibres (∼10μm in diameter) were used to interlay unidirectional (UD), non-crimp fabric (NCF) and 5-Harness satin weave (5H) carbon fibre laminates. The PET and PPS veils remained in a fibrous form and the PA veils melted during the laminate curing process. The results of an end-loaded split test demonstrated significant improvements in the mode-II fracture performance in all cases. In general, interlaying thermoplastic veils was most efficient for toughening the UD laminates, with reduced improvements observed for the 5H and NCF laminates, respectively. The main toughening mechanism of the intact PET and PPS veils was thermoplastic fibre bridging. The melted PA veils mainly improved the fracture toughness of the epoxy at the mid-plane. The different toughening mechanisms of the veils, combined with different fracture mechanisms between the UD, NCF and 5H laminates, resulted in significantly different toughening levels.

Suitable specimen dimensions for the determination of mode II fracture toughness of bonded joints by means of the ELS test

The definition of the specimen dimensions in mode II fracture tests of bonded joints, where large Fracture Process Zones (FPZ) occur, is still an issue to be solved.Several conditions shall be met in order to obtain propagation values in bonded joints testing; stability of the test, full FPZ formation, and avoidance of large deflections and adherend failure.This work presents a methodology that indicates the specimen dimensions needed to obtain propagation values in End Loaded Split (ELS) tests.In addition, suitable specimen dimensions for a wide range of thin bonded joints between CFRP adherends is presented, solving the issue of defining suitable specimen dimensions to test adhesives to ensure steady state propagation.

Guidelines for selecting the dimensions of adhesively bonded end-loaded split joints: An approach based on numerical cohesive zone length

Recent attempts to measure the mode II fracture resistance of structural adhesive joints employing highly toughened adhesives have been hindered by the formation of extensive damage zones ahead of the crack tip. This can lead to strongly rising resistance curves being generated and to the steady-state not being attained. In this paper, a cohesive zone element formulation has been implemented in Abaqus v6.14 via a user element subroutine to determine the length of fracture process zone. It has been demonstrated that the existing analytical expressions cannot predict the length of fracture process zone accurately; hence a formulation for the length of the fracture process zone in an end-loaded split (ELS) joint specimen has been derived using a numerical parametric method. The results obtained from the proposed formulation correlate very well with those from numerical results. The results have allowed guidelines to be developed for selecting the dimensions of an ELS test specimen, comprising fibre-composite substrates bonded with a toughened adhesive, which ensure the attainment of the steady-state and hence a plateau in crack growth resistance curve is ensured. This removes a significant obstacle in the definition of a successful standard method test.

The enhancement of adhesively-bonded aerospace-grade composite joints using steel fibres

Ductile steel fibres with high stiffness of 193 GPa, high strain-to-failure of 20% and diameter of 22 μm were used to enhance fracture toughness of composite adhesive joints. Two nano-toughened structural adhesives with significantly different mechanical and fracture properties were used to bond aerospace-grade composite substrates. Steel fibres were placed in the adhesive layer either longitudinally or transversely to the crack growth direction. Mode-I and mode-II fracture behaviour of the composite adhesive joints were studied using double cantilever beam test and end-loaded split test, respectively. The incorporation of steel fibres significantly increased both mode-I and mode-II fracture toughness, irrespective of the adhesive used. The improvement of mode-I and mode-II fracture energies was more pronounced when the steel fibres were placed transversely to the crack growth direction, due to the increased level of steel fibre bridging.

Interlaminar fracture toughness of 5HS Carbon/PEEK laminates. A comparison between DCB, ELS and mandrel peel tests

The present work focuses on the applicability of the mandrel peel test to quantify the interlaminar fracture toughness of 5 harness satin woven fabric Carbon/PEEK composites. For this purpose, the Mandrel Peel (MP) test was compared to the Double Cantilever Beam (DCB) and End-Loaded Split (ELS) test in terms of experimental procedure and results obtained. The interlaminar toughness of the 5 harness Carbon/PEEK was measured both parallel and perpendicular to the predominant fibre direction at the interface. While stable crack propagation was observed in the ELS test, unstable crack propagation (stick-slip) was observed during both the DCB and the mandrel peel tests. In the case of the mandrel peel test, however, the unstable propagation was immediately arrested by the mandrel, limiting the instability and providing numerous crack re-initiation values per unit of crack length. This effect is expected to increase the statistical relevance of a single test and thereby to increase the reliability of the measured values as compared to DCB tests. A fractographic analysis was performed to study the nature of the crack propagation for the different testing techniques. The mandrel peel test was found to be a potentially plausible alternative to the DCB test for woven fabric reinforced composites.

Inverse End Loaded Split test configuration for stable mode II crack propagation in bonded joint: macroscopic analysis—effective crack length approach

Mode II crack propagation along a bonded joint is investigated using newly proposed Inverse-End Loaded Split experimental configurations. This test configuration allows stable crack propagation all along the crack propagation path. The specimen compliance and strain energy release rate for the new experimental arrangement are derived. An experimental data reduction procedure, based on the effective crack length approach, is also proposed. Two series of experiments are performed to assess the stable nature of the crack propagation and data reduction scheme associated to this new experimental arrangement. In addition to stable crack growth, the experiment may prove his worthiness in the study of crack onset under mode II loading.

A data reduction method based on the J-integral to obtain the interlaminar fracture toughness in a mode II end-loaded split (ELS) test

Various difficulties arise in the data reduction of the end-loaded split (ELS) test. On one hand, a small Fracture Process Zone (FPZ) at the crack front is assumed in the existing mode II end-loaded split test methodologies based on Linear Elastic Fracture Mechanics (LEFM). However, mode II fracture has been reported to involve large FPZ and a fuzzy crack tip. Furthermore, the ELS test, is usually affected by geometrical non-linearities.This work proposes a closed-form solution based on the J-integral to determine the interlaminar fracture toughness in an ELS test. This solution avoids the need to measure the crack length, and is applicable when a large FPZ is present, as occurs in adhesive bonded joints between CFRP. In addition, because the ELS test involves large vertical deflections, a correction of the formulation for large displacements has been implemented.This new methodology has been compared to other methods available in the literature based on LEFM by means of an experimental campaign of delamination tests using unidirectional CFRP specimens in order to make a first validation of the method.

Critical assessment of the mandrel peel test for fiber reinforced thermoplastic laminates

The applicability of the mandrel peel test for thermoplastic composites was investigated experimentally by comparing the fracture toughness to the values obtained by the double cantilever beam (DCB) and end loaded split (ELS) beam test. Two laminates were considered: a unidirectionally carbon-PPS laminate and a carbon-PPS hybrid laminate, in which the interface consisted of a woven fabric and a unidirectionally reinforced ply. The mandrel peel results on the unidirectionally reinforced laminate compared well with the DCB and ELS test results. The fracture toughness of the hybrid interface could not be quantified by the DCB or ELS test, whereas this was possible using the mandrel peel test.

Mode II Fracture Characterization of <i>Pinus Pinaster</i> Wood

This paper describes experimental and numerical studies on the application of the End Notched Flexure (ENF) and End Loaded Split (ELS) tests to mode II wood fracture characterization. In this context, ENF and ELS specimens were used to determine GIIc of a clear Pinus pinaster wood in the RL system, which is the most relevant for structural design. In mode II fracture tests the crack faces are in contact, thus hindering a rigorous visualization of the crack tip. This makes classic methodologies based on crack length measurement during experimental tests inadequate, since they induce significant errors on the mode II fracture properties. To overcome this experimental problem a Compliance Based Beam Method (CBBM) is used. This new data reduction scheme does not require crack length monitoring and includes the effect of the Fracture Process Zone (FPZ) ahead of crack tip. Furthermore, the clamped cross-section rotation of the ELS specimen is also taken into account. In the present work a numerical analysis considering a cohesive damage model was performed with a cohesive damage model in order to validate the application of the CBBM to the experimental results. The results confirmed the adequacy of the CBBM and the applicability of the ENF and ELS tests for mode II wood fracture characterization.

Cohesive zone with continuum damage properties for simulation of delamination development in fibre composites and failure of adhesive joints

A new approach is developed to implement the cohesive zone concept for the simulation of delamination in fibre composites or crack growth in adhesive joints in tension or shear mode of fracture. The model adopts a bilinear damage evolution law, and uses critical energy release rate as the energy required for generating fully damaged unit area. Multi-axial-stress criterion is used to govern the damage initiation so that the model is able to show the hydrostatic stress effect on the damage development. The damage material model is implemented in a finite element model consisting of continuum solid elements to mimic the damage development. The validity of the model was firstly examined by simulating delamination growth in pre-cracked coupon specimens of fibre composites: the double-cantilever beam test, the end-notched flexure test and the end-loaded split test, with either stable or unstable crack growth. The model was then used to simulate damage initiation in a composite specimen for delamination without a starting defect (or a pre-crack). The results were compared with those from the same finite element model (FEM) but based on a traditional damage initiation criterion and those from the experimental studies, for the physical locations of the delamination initiation and the final crack size developed. The paper also presents a parametric study that investigates the influence of material strength on the damage initiation for delamination.

Equivalent crack based analyses of ENF and ELS tests

Equivalent crack based data reduction schemes have been recently proposed for mode II interlaminar fracture tests. These methods avoid the difficulties in monitoring crack propagation throughout the test, as the experimental compliance data is used to calculate the crack position. However, their accuracy has not been demonstrated. In this paper, application of equivalent crack approaches to End-Notched Flexure and End-Loaded Split tests was studied numerically. A cohesive zone damage model based on developed interface finite elements was used. Equivalent crack methods were found to be very accurate for both specimens, while classical beam theory based data reduction schemes underestimated fracture toughness.

Mode II wood fracture characterization using the ELS test

This paper describes experimental and numerical studies on the application of the end loaded split test to mode II wood fracture characterization. A new data reduction scheme, based on the specimen compliance and on the equivalent crack concept, is proposed. The method presents three main advantages relatively to the classical methodologies: it does not require crack measurement during propagation; it accounts for the root rotation at the clamping point and includes the effect of the fracture process zone at the crack tip. The new procedure was numerically validated using a two-dimensional finite element analysis including a cohesive damage model, which allows the simulation of crack initiation and growth. The results demonstrated the good performance of the model and the applicability of the end loaded split test for mode II wood fracture characterization.